Local structural arrangements and corresponding dynamical
effects in relaxor ferroelectrics PbSc1/2Nb1/2O3
and PbSc1/2Ta1/2O3 were studied by Raman
spectroscopy. Polarized Raman spectra of single crystals were
measured at different temperatures in several scattering
configurations. The peaks observed are assigned on the basis of
normal-mode calculations. The effect of small structural
deviations on the atomic vibrations is analysed by calculating
the modes of structural units at various atomic positions. It
is shown that doubling the structural unit is always possible on
a local scale of a few unit cells. The presence of two Raman
peaks originating from the F2u mode shows that
electron-phonon coupling occurs in PbSc1/2B''1/2O3,
thus leading to dynamical off-centre-symmetrical structural
fluctuations. The structural
perturbation consists of the non-coplanarity of Pb and O
atoms in the planes perpendicular to the body diagonal and
deviations of B cations from the centre of BO6 octahedra.
The intensity ratio of the two peaks is sensitive to the degree
of orientational ordering of the lone pairs of Pb atoms within
the Pb-O planes, which is related to the degree of
compositional B'/B'' ordering. The temperature
dependence of the peaks arising from asymmetrical O-B-O
bending, BO3 translation and Pb-localized modes
shows that on cooling different structural changes are
preferential in the two materials studied. In
PbSc1/2Nb1/2O3 the lowering of the temperature
favours the off-centre deviations of the B cations. In
PbSc1/2Ta1/2O3 the temperature decrease
stabilizes the non-coplanarity of the Pb and O atoms in the
planes perpendicular to the body diagonal and enhances the
correlation between the electronic lone pairs of Pb.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.